ABSTRACT
While humans have a unique ability to consider beliefs as true, false, uncertain or undecided, the concept of delusion implies not just that a belief is wrong but that the process of belief formation itself has become disturbed. Given that beliefs refl ect the state of taking something to be the case or regarding it as true (Schwitzgebel, 2014), the assimilation of information to allow us to form beliefs that adequately refl ect reality is a central part of presumably healthy adaptive human cognition. Although suboptimal rationality is the norm in human reasoning, delusions refl ect a diffi culty modifying clearly erroneous or impossible beliefs-particularly beliefs with high emotional salience-resulting in signifi - cant disability. Delusions are therefore typically defi ned as being false, fi xed and unswayable by counter-evidence, although, as we shall see, adequately defi ning delusion is not an easy conceptual task given that a complete defi nition that adequately and coherently distinguishes pathological from non-pathological beliefs by their characteristics turns out to be remarkably diffi cult to formulate (David, 1999). Despite not having a perfect defi nition, the existing criteria mean that, in the clinic, delusions can be diagnosed to an acceptable level of reliability and many scales seek to aid this process by evaluating the dimensions of delusional belief either in its form or content.
